1. Field of the Invention
This invention relates to a radiation image read-out method, wherein light emitted from a front surface of a radiation image storage panel, which is provided with a stimulable phosphor layer, and light emitted from a back surface of the radiation image storage panel are detected respectively.
2. Description of the Prior Art
It has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
As one of technique for photoelectrically detecting the light emitted by a stimulable phosphor sheet, a technique for detecting light emitted from front and back surfaces of a stimulable phosphor sheet and thereby detecting two image signals from the opposite surfaces of the stimulable phosphor sheet has heretofore been known. With the technique for detecting light emitted from front and back surfaces of a stimulable phosphor sheet and thereby detecting two image signals from the opposite surfaces of the stimulable phosphor sheet, for example, a radiation image storage panel is employed, which comprises a transparent substrate (such as transparent film having a thickness falling within the range of 100 xcexcm to 500 xcexcm) and a stimulable phosphor layer overlaid on the front surface side of the transparent substrate. Radiation is irradiated to the radiation image storage panel from its stimulable phosphor layer side, and radiation image information is stored on the stimulable phosphor layer of the radiation image storage panel. Thereafter, irradiation of stimulating rays is performed from the stimulable phosphor layer side of the radiation image storage panel. When the radiation image storage panel is exposed to the stimulating rays, light is emitted from each of the front surface side (i.e., the stimulable phosphor layer side) of the radiation image storage panel and the back surface side (i.e., the transparent substrate side) of the radiation image storage panel. The light emitted from the front surface side of the radiation image storage panel and the light emitted from the back surface side of the radiation image storage panel are respectively detected with photoelectric read-out means, which is located on the front surface side of the radiation image storage panel, and photoelectric read-out means, which is located on the back surface side of the radiation image storage panel. The technique for detecting light emitted from front and back surfaces of a stimulable phosphor sheet and thereby detecting two image signals from the opposite surfaces of the stimulable phosphor sheet is disclosed in, for example, U.S. Pat. No. 4,346,295. In cases where the technique for detecting light emitted from front and back surfaces of a stimulable phosphor sheet and thereby detecting two image signals from the opposite surfaces of the stimulable phosphor sheet is utilized, an addition process can be performed on the image signal components of the two image signals having been detected from the opposite surfaces of the stimulable phosphor sheet, which image signal components represent corresponding pixels on the front and back surfaces of the stimulable phosphor sheet. In this manner, the light collecting efficiency can be enhanced. Further, since noise components are uniformized, the signal-to-noise ratio of the obtained radiation image can be enhanced.
Ordinarily, the conventional radiation image storage panels are provided with thin film as the substrate and are flexible as a whole. Also, in cases where an image read-out operation is performed on the radiation image storage panel, for example, the stimulating rays are deflected in a main scanning direction, and the radiation image storage panel is conveyed in a sub-scanning direction. In this manner, the radiation image storage panel is scanned with the stimulating rays in two-dimensional directions. In cases where a technique for detecting light emitted from one surface alone of a radiation image storage panel (i.e., from only the front surface side of the radiation image storage panel) and thereby detecting only one image signal from the one surface of the radiation image storage panel is employed, no limitation is imposed upon how the back surface of the radiation image storage panel is to be supported. Therefore, in such cases, the back surface of the radiation image storage panel can be supported appropriately and conveyed at the read-out position that is scanned with the stimulating rays. Accordingly, even if the radiation image storage panel has flexibility as described above, the radiation image storage panel can be kept in a stable state at the read-out position by being supported appropriately from the back surface side of the radiation image storage panel at the read-out position. As a result, the image read-out operation can be performed accurately, while the radiation image storage panel is being kept in the state free from any deformation, sway, or the like.
However, it has been found that, in cases where the conventional radiation image storage panel having the flexibility as described above is employed in the technique for detecting light emitted from front and back surfaces of a radiation image storage panel and thereby detecting two image signals from the opposite surfaces of the radiation image storage panel, the problems described below occur. Specifically, in cases where the radiation image storage panel having the flexibility is employed, in order for the image read-out operation to be performed accurately, a conveyance mechanism cannot be kept simple.
More specifically, in cases where the technique for detecting light emitted from front and back surfaces of a radiation image storage panel and thereby detecting two image signals from the opposite surfaces of the radiation image storage panel is employed, it is necessary that a light guide member, or the like, for detecting the light emitted by the radiation image storage panel is located on the back surface side of the radiation image storage panel and at a position close to it at the read-out position. Therefore, the ordinary member for supporting the radiation image storage panel cannot be located on the back surface side of the radiation image storage panel at the read-out position. Accordingly, if the radiation image storage panel having the flexibility is employed in such cases, a complicated mechanism for preventing the radiation image storage panel from being deformed due to deflection by gravity must be utilized at the read-out position.
The primary object of the present invention is to provide a radiation image read-out method, wherein an operation for detecting light emitted from front and back surfaces of a radiation image storage panel and thereby detecting two image signals from the opposite surfaces of the radiation image storage panel is capable of being performed accurately.
Another object of the present invention is to provide a radiation image read-out method, wherein an operation for detecting light emitted from front and back surfaces of a radiation image storage panel and thereby detecting two image signals from the opposite surfaces of the radiation image storage panel is capable of being performed accurately and efficiently.
The present invention provides a radiation image read-out method, wherein a radiation image storage panel, which is provided with a stimulable phosphor layer and on which a radiation image has been stored, is exposed to stimulating rays, which cause the radiation image storage panel to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, and the light emitted from a front surface of the radiation image storage panel and the light emitted from a back surface of the radiation image storage panel are photoelectrically converted into image signals respectively,
the method comprising employing a radiation image storage panel, which has rigidity, as the radiation image storage panel.
In the radiation image read-out method in accordance with the present invention, the radiation image storage panel may comprise a transparent substrate having rigidity and the stimulable phosphor layer overlaid on the transparent substrate.
Also, the radiation image read-out method in accordance with the present invention should preferably be modified such that, in cases where the radiation image storage panel comprises a transparent substrate, which constitutes the back surface of the radiation image storage panel, and the stimulable phosphor layer, which is overlaid on the front surface side of the transparent substrate, and the light emitted from the back surface of the radiation image storage panel is guided through a light guide member, which has a light input face located close to the back surface of the radiation image storage panel, into photoelectric conversion means,
the light guide member is located so as to satisfy the formulas:
txe2x89xa61/(2 tan xcex8)xe2x88x92s
sin xcex8=n2/n1
in which t represents the thickness of the transparent substrate, s represents the distance between the back surface of the radiation image storage panel and the light input face of the light guide member, 1 represents the width of the light input face of the light guide member, n2 represents the refractive index of air, and n1 represents the refractive index of the transparent substrate.
Further, in the radiation image read-out method in accordance with the present invention, an image read-out operation for detecting the light emitted by the radiation image storage panel is a panel-securing image read-out operation, in which the radiation image storage panel is kept stationary, and read-out means is moved with respect to the radiation image storage panel.
The term xe2x80x9cradiation image storage panel having rigidityxe2x80x9d as used herein means the radiation image storage panel having a level of rigidity such that the amount of deflection of a portion of the radiation image storage panel by gravity, which portion extends over the distance (ordinarily, the distance falling within the range of approximately 50 mm to approximately 200 mm) between radiation image storage panel support members (in the cases of the embodiment described later, endless belts 19a and 19b) located on both sides of the position that is scanned with the stimulating rays, does not adversely affect the light guiding efficiency. By way of example, the level of rigidity, which does not adversely affect the light guiding efficiency, is such that, in cases where the radiation image storage panel is supported in a cantilever beam-like form, in which a portion of the radiation image storage panel is projected by a length of 50 mm from a support point and the end of the projected portion of the radiation image storage panel is not supported, the amount of downward deflection of the end (the free end) of the projected portion of the radiation image storage panel is at most 2 mm, and is preferably at most 1 mm. (For example, in cases where the radiation image storage panel is supported and conveyed by an endless belt, the support point represents the foremost support position of the endless belt with respect to the direction of conveyance. In cases where the radiation image storage panel is nipped between a pair of upper and lower rollers and is supported and conveyed in this state, the support point represents the position that is nipped and supported by the rollers.) In order for such a level of rigidity to be imparted to the radiation image storage panel, for example, the transparent substrate having the level of rigidity may be employed. In such cases, a glass sheet, a plastic sheet (e.g., a polycarbonate sheet or an acrylic sheet), or the like, which is transparent with respect to the light emitted by the radiation image storage panel and which has a thickness falling within the range of approximately 0.5 mm to approximately 5 mm, may be employed as the transparent substrate.
With the radiation image read-out method in accordance with the present invention, instead of a conventional radiation image storage panel with flexibility being employed, the radiation image storage panel having rigidity is employed as the radiation image storage panel. Therefore, in cases where the operation for detecting light emitted from the front and back surfaces of the radiation image storage panel and thereby detecting two image signals from the opposite surfaces of the radiation image storage panel is to be performed, in which the radiation image storage panel cannot be sufficiently supported from the back surface at the time of the image read-out operation, there is no risk that the radiation image storage panel will be deflected at the read-out position and will be swayed at the read-out position during the conveyance of the radiation image storage panel. Accordingly, a failure in image readout due to deflection and swaying can be prevented from occurring, and the image read-out operation can be performed accurately.
With the radiation image read-out method in accordance with the present invention, wherein the radiation image storage panel comprises the transparent substrate having rigidity and the stimulable phosphor layer overlaid on the transparent substrate, rigidity can be easily and sufficiently imparted to the radiation image storage panel.
In cases where the substrate of the radiation image storage panel is constituted of the transparent substrate having rigidity, such as a glass sheet, in order for light guiding to be performed efficiently, the critical angle at the transparent substrate should be taken into consideration, and the light guide member on the back surface side of the radiation image storage panel should be located appropriately. Therefore, as described above, in the radiation image read-out method in accordance with the present invention, the critical angle at the transparent substrate should preferably be taken into consideration, and the light guide member should preferably be located so as to satisfy the formulas shown above, in which t represents the thickness of the transparent substrate, s represents the distance between the back surface of the radiation image storage panel and the light input face of the light guide member, 1 represents the width of the light input face of the light guide member, n2 represents the refractive index of air, and n1 represents the refractive index of the transparent substrate. With the radiation image read-out method in accordance with the present invention, in which the light guide member is located in this manner, the light emitted by the stimulable phosphor layer, which light passes through the transparent substrate and emanates from the back surface of the radiation image storage panel, can be guided efficiently with little loss. In this manner, the image read-out operation can be performed, in which the light guiding efficiency is enhanced with little loss by the consideration of the critical angle at the transparent substrate.